While significant progress has been made toward identifying effective interventions for preschool-age children with autism (National Research Council, 2001), few scientifically rigorous studies have compared active ingredients of these interventions or examined outcomes focused on core deficits. To address these areas of need, this collaborative, multi-site project combines the expertise of investigators experienced in randomized controlled clinical trials (RCTs), in the study of core deficits in young children with autism, and in data management and analysis of multi-site clinical trials.